The main structural elements of a known mobile communications system are shown in FIG. 1. The figure shows a mobile services switching centre (MSC), a base station controller (BSC), a base transceiver station (BTS), a mobile station (MS) and a network management system (NMS). The network also typically comprises several inter-connected mobile services switching centres (MSC), of which only one is shown in the figure for the sake of clarity. The mobile station system is connected to a fixed telephone network, e.g. a public switched telephone network (PSTN), or an integrated services digital network (ISDN), through a mobile services switching centre MSC. Typically, several base station controllers BSC hierarchically under the MSC are connected to each mobile services switching centre MSC. Several base transceiver stations BTS hierarchically under the BSC are typically connected to each base station controller BSC. The base transceiver stations can set up connections with the mobile stations MS by way of channels through a so-called air interface. For the sake of simplicity, the figure shows only one base station controller BSC, one base transceiver station BTS and one mobile station MS. The network management system NMS may be used for controlling the operation of network elements, and e.g. for changing the network configuration.
The air interface between base transceiver stations and mobile stations can be divided into channels in several different ways. Known methods are at least TDM (Time Division Multiplexing), FDM (Frequency Division Multiplexing) and CDM (Code Division Multiplexing). The band available in TDM systems is divided into successive time slots. A certain number of successive time slots forms a periodically repeating time frame. The channel is defined by the time slot available in the time frame. In FDM systems, the channel is defined by the available frequency, while in CDM systems it is defined by the spread code to be used. Combinations of the methods of division mentioned above are also used. E.g. the known GSM system uses FDM/TDM division, whereby frequency and time slot determine the channel.
To obtain sufficient capacity in the limited frequency band of the mobile communications system, the channels in use must be used several times. For this reason, the coverage area of the system is divided into cells. Each cell has its own geographical area or cell area. Each cell has a base transceiver station serving the mobile stations located within the cell area. If channels having the same frequency are reused in cells located too close to each other, the connections using these channels will begin interfering with one another. The channel is caused interference not only by reuse of the own channel but also by adjacent channels used nearby, because adjacent channels are always slightly overlapping to save the frequency band. To maximise the capacity the channels must be reused in cells as close to one another as possible, however, so that the carrier to interference ratio CIR will allow an adequate connection quality. The distance at which the same channel may be reused so that the CIR remains acceptable is called the interference distance, while the distance at which the same channel is reused is called the reuse distance. Since the CIR is a function of reuse distance and transmission power, the carrier to interference ratio may be reduced in a cellular system so as to improve the quality of the connection by increasing the distance between base transceiver stations or by using dynamic control of the transmission power.
In channel allocation the objective is to allocate channels for desired connections, which channels may all be used at the same time while the quality of signals remains acceptable. Methods of channel allocation are at least FCA (Fixed Channel Allocation), DCA (Dynamic Channel Allocation) and HCA (Hybrid Channel Allocation) obtained as a combination of these. The various methods are described quite thoroughly in a publication by I. Katzela, M. Naghshineh: "Channel Assignment Schemes for Cellular Mobile Telecommunication Systems: A Comprehensive Survey", IEE Personal Communications, June 1996.
In fixed channel allocation, a set of channels is assigned for each cell according to some reuse scheme. Typically, the channels are reused in every 9.sup.th or in every 12.sup.th cell. Simplicity is an advantage of this method, but it suffers from an inability to adapt to traffic situations and to changes in the number of mobile subscribers in the cells. In addition, to obtain a good result the method necessarily requires frequency planning based on signal drop-outs which are difficult to predict.
In dynamic channel allocation, all channels are in a joint "channel pool". Channels are taken dynamically from the pool for use in the cell for new calls or for channel exchanges in the cell as calls arrive in the system. It is ensured at the same time that a minimum CIR ratio is preserved. Thus there is no fixed relation between channels and cells, but any channel can be selected by any cell, provided that the carrier to interference ratio is acceptably low. Advantages of this method are great flexibility and an ability to adapt to changing traffic, but on the other hand it is more inefficient than fixed channel allocation, if the load is very high.
In hybrid channel allocation, the available channels are divided into fixed and dynamic channels, of which the fixed channels are assigned FCA typically for use by certain cells while the dynamic channels are assigned DCA typically for use by all users. The cells always attempt primarily to use their fixed frequencies.
The dynamic channel allocation methods may be divided into those based on measurements of channel carrier to interference ratios and those based on knowledge of the allocation situation.
In the methods based on knowledge of the allocation situation, the carrier to interference ratios caused by operating connections are estimated and such areas around base transceiver stations are defined within which reuse of a channel in use at the base transceiver station or use of channels adjacent to the channel would cause excessive interference. Allocation of the concerned channels is prevented in these areas.
Dynamic channel allocation methods based on measurements of channel carrier to interference ratios define the strength of the channel candidate's existing interfering signal. If the concerned channel were made available to the connection to be set up, this signal existing on the channel would cause interference to the connection. The channel carrier to interference ratio is thus defined by measuring, whereby direct information is obtained about the carrier to interference ratio to be optimised. As the carrier to interference ratio may vary even strongly e.g. due to DTX (Discontinuous Transmission), the signal measurements used for defining the carrier to interference ratio must be averaged to obtain sufficient reliability in practice.
Channel allocation is studied as an example in the situation shown in FIG. 2, where two mobile stations MSA and MSB request a channel of nearby base transceiver stations BTSA and BTSB. A channel is requested first by MSA and then a little later by MSB. The figure shows base transceiver stations and cells formed around these. The cells depict an area where mobile stations seek connection with the base transceiver station of the cell.
In the example, the channel allocation method is embodied in base transceiver stations BTS. The channel allocation algorithm used as an example is the MCIR (Maximum Signal to Noise Interference Ratio) method, which is a subclass of the dynamic channel allocation methods based on signal level measurement. In this method such channels are sought on which as good a signal to noise ratio as possible is achieved in the up-link direction from mobile station to base transceiver station. A method of this type can be used directly e.g. in an existing GSM system where mobile stations do not perform measurements of the carrier to interference ratio of channel candidates.
Channel allocation for mobile station MSA is studied first. Mobile station MSA receives an instruction from its user to set up a call connection. The mobile station requests a channel of the network for setting up the said connection by sending a channel request message to base transceiver station BTSA which receives the request.
Base transceiver station BTSA receives the channel request of the mobile station and begins performing the channel allocation method shown in FIG. 3. The base transceiver station constantly monitors the signal levels of all busy channels in the up-link direction (point 311). Based on the monitoring measurements, the channels are placed in an order of best to worst, according to a certain norm (point 312). Since signal levels may vary even strongly as a function of time, e.g. due to discontinuous transmission DTX used by mobile stations, the measurement results must always be averaged. As a norm of channel carrier to interference ratio the case shown as an example uses a five-second moving average of signal level measurements performed on the channel.
Having received the channel request, point 302, the base transceiver station BTSA picks up that channel from the list of channels received by it at point 312 on which, according to performed measurements, the best possible CIR will be achieved, point 303. This channel is allocated for use by the connection to be set up (point 304).
FIGS. 4 and 5 show how introduction of the channel allocated for MSA will affect the carrier to interference ratio of channel K at base transceiver station BTSB. FIG. 4 shows interfering signals on channel K of base transceiver station BTSB as a function of time. According to FIG. 4, there is hereby only one cause of interference, interference 1, in the signal measured by the base transceiver station on channel K. The connection started at moment T=12:58:10 in FIG. 4 between base transceiver station BTSA and mobile station MSA begins causing a new interference to channel K (interference 2).
The total interference on channel K of base transceiver station BTSB is shown as a function of time in FIG. 5. Besides, FIG. 5 shows a one-second moving average of total interference 5 which the base transceiver station uses for determining the CIR value of the channel. Before channel K is put into use for the connection between mobile station MSA and base transceiver station BTSA, channel K obtains the best CIR value also at base transceiver station BTSB.
Allocation of a channel for the connection between mobile station MSB and base transceiver station BTSB is studied next. As before, this channel allocation decision is also based on a measurement of the channel carrier to interference ratio.
For an incoming call, MSB requests a channel of base transceiver station BTSB, which is located near base transceiver station BTSA. Base transceiver station BTSB receives the channel request (FIG. 3, point 302) and begins at moment 12:58:11 looking at point 311 for a channel for the connection to be set up based on the channel carrier to interference ratios it has measured. After reception of the channel request (FIG. 3, point 302), the channel (point 303) which is best according to CIR values is picked up from the list of channels arranged according to five-second averages of CIR values formed at point 312. Even though the carrier to interference ratio of channel K has risen in reality due to the introduction of the channel at base transceiver station BTSA, the risen carrier to interference ratio is not yet seen in the values used by the channel allocation algorithm of base transceiver station BTSB due to the delay in averaging. Under these circumstances, the algorithm will allocate channel K also for the connection between base transceiver station BTSB and mobile station MSB. Since the same channel is now used in two connections which are geographically close to one another, the quality of the connection is poor in connections both between MSA and BTSA and between MSB and BTSB.
Based on the above, it is easy to understand that there are problems due to averaging of measurement results in DCA methods based on measurements of channel carrier to interference ratios. When a channel is allocated between a base transceiver station and a mobile station and a call connection is started on the channel, the system's carrier to interference ratio will increase near the base transceiver station on this channel and on its adjacent channels. However, due to averaging of interference measurement results, the increase in carrier to interference ratio is not noticed at once, but only after a certain delay in averaging. The risk thus exists that during the said delay a channel will be allocated for use by some other connection so that the first connection and the second connection will interfere excessively with one another. It is hereby probable that at least the channel of one connection must be exchanged, which will result in signalling loading the network. In the worst case the connection may even be cut off.
Brief summary of the invention The present invention aims at eliminating or reducing the state-of-the-art problems presented above. This objective is achieved by a method which is defined in the independent claims.
The inventive idea is, based on limitation information which changes as time passes, to limit allocation of those channels the introduction of which would cause mutual interference with connections already set up. The limiting information is maintained separately for each base transceiver station and information is collected to it on the channel allocation situation near the base transceiver station. Based on the limiting information, channel allocation is limited during the time when measurement results obtained from interference measurements are incorrect due to delays in measurements and averaging. Channel allocation may be limited e.g. by preventing it altogether or by limiting the maximum power made available to the connection or the number of time slots to be used on the channel. In this way one avoids allocating such a channel for use by a connection to be set up which still seems free of interference due to an averaging delay but the carrier to interference ratio of which has already risen in reality.
In a preferred embodiment of the invention the time of limitation of allocation is essentially equal to the time of interference measurement averaging. Hereby the method will at once resume the channel allocation procedure based on state-of-the-art signal level measurement when it is certain that the carrier to interference ratio is no longer considered too low due to a delay in the averaging of measurement results.